High vacuum mass analyser apparatus



Jan. 7, 1964 A. B. FRANCIS ETAL HIGH VACUUM MASS ANALYSER APPARATUS Filed July 31, 1961 FlG.l

INVENTORS ARTHUR B.FRANCIS JOHN C. HELMER ALFRED E. BARRINGTON flzw iTTORNEY United States Patent Varian Associates, Palo Alto, Calif., a corporation of California Filed July 31, 1961, Ser. No. 128,032 6 Claims. (Cl. 25041.9)

This invention relates to apparatus for high vacuum mass analysis. More particularly, the present invention relates to apparatus for high vacuum mass analysis utilizing a mono-voltaic positive ion source.

Prior high sensitivity vacuum mass analysers have for the most part been bulky and expensive because of the necessity for large magnetic fields, high voltage supplies, complex ion sources, etc. Furthermore, many conventional mass analyser devices have been unable to operate at extremely low pressures.

It is therefore the object of the present invention to provide a very sensitive device which will function as a mass analyser at extremely low pressures.

One feature of the present invention is a unique arrangement of an ion gun, a focusing lens, a magnetic deflector and an electrostatic deflector to produce an improved high vacuum leak detector.

Another feature of the present invention is the provision in a gas discharge device of a novel pole piece arrangement to provide an improved high vacuum ion source.

Still another feature of the present invention is the provision of a gas discharge ion gun whose anode and cathode electrodes may be supplied with variable voltages to thereby produce an extremely flexible positive ion source.

These and other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein,

FIG. 1 is an end view of one embodiment of the present invention,

FIG. 2 is an enlarged partial cross sectional top view taken along the lines 2--2 of FIG. 1,

FIG. 3 is a cross sectional side view of the positive ion gun of the present invention taken along the lines 33 of FIG. 2,

FIG. 4 is a cross sectional view of the magnetic deflector portion of the present invention taken along the lines 4-4 of FIG. 2,

FIG. 5 is a cross sectional view of the electrostatic deflector of the present invention taken along the lines 55 of FIG. 2,

FIG. 6 is a cross sectional view of the ion collector of the present invention taken along the lines 66 of FIG. 2, and

FIG. 7 is an enlarged cross sectional View of another ion gun and electrostatic focuser embodiment of the present invention.

Referring now to FIGS. 1 through 6 there is shown a positive ion gun 11 which directs a beam of positive ions 12 through the vacuum tight envelope 13 after which it is collected by the ion collector 14. The vacuum envelope 13 includes an electrostatic focusing portion 15, a magnetic deflector portion 16 and an electrostatic deflector portion 17.

As shown in FIGS. 1-3 the ion gun 11 has a hollow cylindrical anode electrode 18 whose open ends are opposite and spaced from the solid portions of a back cathode plate 19 and a front cathode plate 21. A hollow cylindrical casing 22 is sealed between the cathode plates 19 and 21 so as to form an ion gun vacuum compartment 23 for the anode electrode 18. Connected to the anode electrode 18 is the lead-in conductor 24 which passes through and is insulated from the cylindrical casing 22 by the ceramic insulator assembly 25. A U-shaped magnet 26 is connected to the cathode plates 19 and 21 which are made of magnetic material, as for example magnetic stainless steel, and thus serve as pole pieces to direct a magnetic field through the open ends of the cylindrical anode electrode 18. An annular mounting flange 27 is attached to the front cathode plate 21 which is also provided with a plurality of circularly arranged apertures 28 which provide gas communication passageways into the vacuum compartment 23. The back cathode plate 19 is also provided with a plurality of circularly arranged apertures 29 which provide gas communication paths between the vacuum compartment '23 and the vacuum envelope 13. The back cathode plate 19 is further provided with a frustrum shaped aperture having its larger opening facing the vacuum envelope 13 and its central axis along the central axis of the cylindrical anode electrode 18. The frustrum shaped aperture 30 provides a passageway into the Vacuum envelope 13 for the ion beam produced in the vacuum compartment 23 of the ion gun 11.

The operation of the ion gun is as described and claimed in co-pending US. application Ser. No. 120,598.

The electrostatic focusing portion of the vacuum envelope 13 has a hollow cylindrical focusing electrode 31 positioned within a larger hollow cylindrical casing 32 which is grounded and vacuum sealed to the back cathode plate 19. Attached to the focusing electrode 31 is a conductor lead-in 33 which passes through and is insulated from the casing 32.

The magnetic deflector portion 16 (see FIGS. 2 and 4) of the vacuum envelope 13- has a pair of spaced apart arced concentric plates 34 and 35 whose curved edges are sealed to a pair of fiat cover plates 36 and 37 to form a curved magnetic deflector chamber 38. One end of the deflector compartment 38 is closed by a centrally apertured back end plate 39 and its other end is closed by a centrally apertured front end plate 41 which is sealed to the focusing electrode casing 32. The magnetic pole pieces 40, shown in phantom in FIG. 4, are positioned adjacent the cover plates 36 and 37 so as to produce a transverse magnetic field B within the magnetic deflector chamber 38. A plurality of spaced apart baffle plates 42 are attached to and extend approximately /3 of the distance between the arced magnetic deflector plates 34 and 35.

The electrostatic deflector portion 17 (see FIGS. 2, 5 and 6) of the vacuum envelope 13 has grounded side walls formed by an arced plate 43 having one radius of curvature and another concentric arced plate 44 having a smaller radius of curvature. The arced plates 43, 44 are sealed at their curved edges by a pair of cover plates 45 and 46. One end of the electrostatic deflector 17 is sealed to the apertured back end plate 39 of the magnetic deflector 13 while its other end is closed by an end plate 47 having an aperture 48 covered by the voltage shielding screen 54. A curved deflector electrode plate 49 is positioned between and concentric to the arced deflector plates 43 and 44 to form with the smaller radius deflector plate 44 a curved deflector passage 50 which is aligned with the central aperture of magnetic deflector back end plate 39. The curved deflector electrode 49 is supported by a pair of lead-in conductors 51 and 52 which pass through and are insulated from one of the electrostatic deflector cover plates 45 by ceramic insulator assemblies 53.

The ion collector plate 14 is positioned opposite the aperture 48 in the electrostatic deflector end plate 47 and is enclosed by a partially cutaway hollow cylindrical casing 55. The cutaway edges of casing 55 terminate on and are sealed to the electrostatic deflector end plate 47 and arced side Wall plate 44 respectively. One end of the cylindrical ion collector casing 55 is closed by an end plate 56 and the other end by an end plate 57. Connected to the ion collector electrode 14 is an ion current lead-in conductor 58 which passes through and is insulated from the end plate-57 by a ceramic insulator assembly 59.

In the operation of the present invention the flange 27 would be attached to a mating flange (not shown) on a vacuum system to be leak tested. The pumping mechanism of the vacuum system being tested would-then be used to evacuate the ion gun 11 and the vacuum envelope 13 through the gas passageways 28 and 29 in the ion gun cathode electrodes 19 and 21. When the pressure Within theion' gun 11 has been sufliciently reduced as, for example, to lO mm. of Hg a positive potential is applied to the anode electrode 18. Thereafter in steady state operation secondary electrons are emitted from the grounded cathode electrodes 19 and 21 and are attracted to the anode electrode 18 because of the positive potential thereon but are constrained by the magnetic field from directly reaching the anode 18. However, upon making a collision with a gas molecule the electron loses energy and is thereby able to move closer to the anode electrode eventually being collected on the anode 18. In addition, some of these collisions between electrons and gas molecules are ionizing collisions which liberate positive gas ions and secondary electrons which are added to the discharge.

The positive ions liberated in the anode electrode 18 are focused toward the cathode plates 19 and 21. Con sequently one-half of the' positive ions produced will be collected by the cathode plate 21 while the other half will be directed toward the cathode plate 19. A substantial fraction of this latter one-half will pass through the frustrum aperture 30 to form a beam of positive ions 12. Under proper operating conditions the ion beam 12 then passes through the focusing electrode 31 where it is focused by the electrostatic field lines therein, These field lines result from applying a voltage to the focusing electrode 31 through the lead-in conductor 33.

The focusing portion 15also functions as a discriminator by repelling ions having a voltage less than that applied to the focusing electrode 31, thus reducing the energy spread of the ion beam 12.

The focused ion beam then passes through the central aperture of the magnetic deflector front end plate 41 and into the region of transverse magnetic field B produced by the pole pieces 40. The magnetic field deflects each individual positive ion in the ion beam 12 along a radius of curvature dependent on the individual ions (5 ratio However, since the ion beam produced by the properly operated ion gun 11 comprises primarily singly charged mono-voltaic ions they will be primarily deflected according to their mass alone. The strength of the magnetic field produced by the magnetic deflector pole pieces 40 is such that all ions of a given mass in the ion beam 12 will be deflected upon a single path so as to pass through the central aperture of the magnetic deflector back end plate 39. Ions of other than said given mass will be deflected along other paths so as to be stopped by the projecting batfle plates 42 the magnetic deflector side walls 34, 35 or the solid portions of the magnetic deflector back end plate 39.

The magnetically deflected ions which pass through the aperture in the magnetic deflector back end plate 39 then enter the electrostatic deflector passage 50 in which a transverse electrostatic field is obtained by applying a voltage to the electrostatic deflector electrode plate 49 voltage mass charge through its input conductors 51 and 52. The electrostatic field deflects ions along radii of curvature depend cut on their voltage level and the strength of the field is such as to deflect the mono-voltaic ions along a path which terminates on the collector electrode 14. However, the limited number of ions having other than the mono-voltage which have passed through the magnetic deflector back end plate 39 will be stopped by the side walls of the electrostatic deflector 17. These ionsinclude those whose quantity caused them to be rejected in the magnetic analyser but which have collided with solid portions of the magnetic deflector 16 and been deflected through the aperture in the deflector back end plate 39 and also those having the same ratio as that of the mono-voltaic ions of given mass.

The positions of the magnetic deflector portion loand the electrostatic deflector 17 in the. vacuum envelope 13.

can-also be reversed. However, the embodiment shown in FIG. 2 is a preferred embodiment having certain ad vantages. The ion beam 12 will normally consist of many more ions having an undesirable mass than those having an undesirable voltage. Therefore, the magnetic deflector portion 16 which sepa'rates according to mass will reject a greater number of undesirable ions than will the electrostatic deflector 17. Thus, in the embodiment of FIG. 2 a smaller number of undesirable ions will reach the portion of the vacuum envelope 13 immediately preceding the ion collector 14 than would if the positions of magnetic deflector 16 and electrostatic deflector 17 were reversed. The number of undesirable ions which reach the'ion collector 14 because of col lision paths is thereby reduced. Also, the width of ion beam 12 in the direction of the magnetic field of mag-, netic deflector 15 steadily increases after the beam leaves the focusing electrode 31. Therefore, placement of the magnetic deflector a greater distance away from the focusing portion 15 would require a larger magnetic gap if part of the beam is not to be cut off.

Thus, the device of this invention can be used as a leak detector by establishing the magnetic field in the magnetic deflector portion 16 and the electrostatic field in the electrostatic deflector 17 so that only mono-voltaic helium ions will traverse through the entire vacuum envelope 13 to be collected by the collector plate 14. A stream of helium gas would then be sequentially applied to the tested vacuum systems various external joints which might be the subjects of a leak. The magnitude of ion current collected by the collector 14 would then be observed on a meter (not shown) and the appearance of an increased ion current would indicate an increased quantity of helium within the vacuum system and thus establish the position of the leak. An experimental model of this leak detector has exhibited extremely high sensitivity e.g. detection of partial helium pressures below 2 l0 mm. Hg, Gases other than helium, for example, hydrogen, could be used in the same way.

Also, the device could be used as a mass spectrometer by providing, for example, a sweeping magnetic field'in the magnetic deflector portion 16 and analyzing the ion current spectrum collected by the ion collector 14.

The ion gun embodiment shown in FIG. 7 is identical to that shown in FIG. 2 except that a separate cathode assembly 61 is provided. The cathode assembly 61 includes a solid cathode plate 63 positioned between and spaced from the cylindrical anode 18 and the flanged pole piece 2T'and an apertured cathode plate 64 positioned between and spaced from the anode electrode 18 and the pole piece 19 containing the frustrum shaped aperture 39. The aperture 65 of the apertured cathode 64 is aligned with the central axis of the cylindrical anode l8 and the frustrum shaped aperture 3t). Attached to a conductor bar 66 which supports the cathode plates 63 and 64 is a cathode lead-in conductor 67 which extends through and is insulated from the cylindrical ion gun casing 22.

The ion gun embodiment of FIG. 7 has the advantage of allowing a variation in the applied cathode voltage. This permits, for example, changing the applied anode voltage while maintaining a constant anode-cathode potential difierential by effecting a corresponding cathode voltage change. Such a feature is desirable since many characteristics of the ion beam produced are dependent upon this anode-cathode potential difference. Thus, these ion beam characteristics can be maintained constant for any anode voltage necessary to produce an ion beam of a desired energy level.

Greater sensitivity and ease of ion current detection can be obtained for the device of FIG. 2 by providing an A.C. modulated ion beam. This can be done by, for example only, connecting the secondary winding 71 of a transformer 72 between the anode lead-in 24- and a grounded DC. power supply 73 and connecting an A.C. modulator '74 to the primary winding 75 of the transformer 72. Similarly the voltage applied to the focusing electrode 31 or the electrostatic deflector electrode 49 could be modulated to produce a modulated ion beam. Also, in the gun embodiment of FIG. 7 the anode and cathode voltages could be synchronously modulated to produce an A.C. modulated ion beam while maintaining other beam characteristics constant.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high vacuum apparatus comprising, an ion gun having a hollow open ended anode and cathode structure spaced from and opposite the open ends of said anode, means for providing a magnetic field between said anode and cathode, said anode and cathode being adapted to sustain a gas discharge so as to direct a rectilinear beam of positive ions, a vacuum envelope providing a path for the passage of the ions produced by said ion gun, said vacuum envelope including a magnetic deflector portion with an entrance opening directly in line with the rectilinear positive ion beam, said magnetic deflector portion adapted to contain a magnetic field for deflecting the ions in the ion beam produced by said ion gun out of their rectilinear path along paths dependent upon their ratios, an ion collector electrode positioned in said vacuum envelope so as to collect ions of a given ratio, means for stopping the ions of other than said given ratio, said means for providing a magnetic field between said anode and cathode electrodes including a, magnetic pole piece positioned opposite each open end of said anode electrode, a casing means sealed between said pole pieces to form a vacuum compartment for said anode electrode, said vacuum envelope being vacuum sealed to one of said pole pieces, and said pole piece sealed to said vacuum envelope having an aperture which allows the passage of the ion beam into said vacuum envelope.

2. The apparatus according to claim 1 wherein the other of said pole pieces is adapted for connection to a vacuum system, and each of said pole pieces has passageways which provide gas communication paths between said vacuum system and vacuum envelope through said casing means.

3. Apparatus according to claim 1 including means for focusing the ion beam positioned in said vacuum envelope between said ion gun and said magnetic deflector portion, and an electrostatic ion separating means positioned in said vacuum envelope between said magnetic deflector portion and said ion collector electrode.

4. Apparatus according to claim 2 including means for focusing the ion beam positioned in said vacuum envelope between said ion gun and said magnetic deflector portion, and an electrostatic ion separating means positioned in said vacuum envelope between said magnetic deflector portion and said ion collector electrode.

5. Apparatus according to claim 4 wherein said cathode electrodes are positioned between said anode electrode and said pole pieces, cathode conductor means for applying a voltage to said cathode electrodes, anode conductor means for applying a voltage to said anode electrode, and means for insulating each of said anode and cathode conductor means from said casing means.

6. A high vacuum apparatus comprising an ion gun having a hollow open ended anode and cathode 'electrodes spaced from and opposite the open ends of said anode, means for providing a magnetic field between said anode and cathode electrode, said anode and cathode electrodes being adapted to sustain a gas discharge so as to direct a rectilinear beam of positive ions, a vacuum envelope providing a path for the passage of the ions produced by said ion gun, said vacuum envelope including a magnetic deflector portion with an entrance opening directly in line with the rectilinear positive ion beam, said magnetic deflector portion adapted to contain a mag netic field for deflecting the ions in the ion beam produced by said ion gun out of their rectilinear path along paths dependent upon their ratio including means for focusing the ion beam positioned in said vacuum envelope between said ion gun and said magnetic deflector portion, and an electrostatic ion deflecting means positioned in said vacuum envelope between said magnetic deflector portion and said ion collector electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,947,868 Herzog Aug. 2, 1960 OTHER REFERENCES Pauli et al.: Nuclear Instruments 2 (1958), pp. 227- 236; North-Holland Publishing Co., Amsterdam. (Pulsed High-Intensity Ion Source), Part II. 

1. A HIGH VACUUM APPARATUS COMPRISING, AN ION GUN HAVING A HOLLOW OPEN ENDED ANODE AND CATHODE STRUCTURE SPACED FROM AND OPPOSITE THE OPEN ENDS OF SAID ANODE, MEANS FOR PROVIDING A MAGNETIC FIELD BETWEEN SAID ANODE AND CATHODE, SAID ANODE AND CATHODE BEING ADAPTED TO SUSTAIN A GAS DISCHARGE SO AS TO DIRECT A RECTILINEAR BEAM OF POSITIVE IONS, A VACUUM ENVELOPE PROVIDING A PATH FOR THE PASSAGE OF THE IONS PRODUCED BY SAID ION GUN, SAID VACUUM ENVELOPE INCLUDING A MAGNETIC DEFLECTOR PORTION WITH AN ENTRANCE OPENING DIRECTLY IN LINE WITH THE RECTILINEAR POSITIVE ION BEAM, SAID MAGNETIC DEFLECTOR PORTION ADAPTED TO CONTAIN A MAGNETIC FIELD FOR DEFLECTING THE IONS IN THE ION BEAM PRODUCED BY SAID ION GUN OUT OF THEIR RECTILINEAR PATH ALONG PATHS DEPENDENT UPON THEIR 